Pump with automatic prime device

ABSTRACT

A motor-driven pump with a priming device which is automatically disengaged from the motor when the pump is primed. In the preferred embodiment a submerged centrifugal booster pump and electric motor unit has a built-in hydroturbine priming pump driven by the motor through a spring biased clutch which is disengaged whenever the booster pump is primed and developing sufficient liquid pressure to overcome the spring bias on the clutch.

United States Patent [72] Inventor Gerald A. Nicastro Cleveland, Ohio[21] Appl. No. 828,321 [22] Filed May 27. 1969 [45] Patented June 15,1971 [73] Assignee TRW inc.

Cleveland, Ohio [54] PUMP WITH AUTOMATIC PRIME DEVICE 11 Claims, 6Drawing Figs.

[52] 0.8. CI 417/202, 4l7/216,4l7/316 [51 Int. Cl F04d 9/00, F04b 23/ 14 [50] Field of Search 417/202, 203, 316, 319, 223, 201, 216; 103/1 13;192/86, 91 A [56] References Cited UNITED STATES PATENTS 1,107,5338/1914 Lorenz 417/202 2,292,896 8/1942 Morgan 103/113 2,666,393 l/l954Troeger et a1. 103/113 3,031,974 5/1962 Edwards 103/88 3,145,816 8/1964Lorean et a1 192/91A 3,299,820 l/l967 Campbell 103/113 FOREIGN PATENTS637,587 10/1936 Germany 417/202 810,845 3/1959 Great Britain.. 417/202430,421 2/1948 ltaly 417/202 Primary ExaminerHenry F RaduazoAttorneyl-lill, Sherman, Meroni, Gross & Simpson ABSTRACT: Amotor-driven pump with a priming device which is automaticallydisengaged from the motor when the pump is primed. 1n the preferredembodiment a submerged centrifugal booster pump and electric motor unithas a built-in hydroturbine priming pump driven by the motor through aspring biased clutch which is disengaged whenever the booster pump isprimed and developing sufficient liquid pressure to overcome the springbias on the clutch.

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SHEET 1 OF 2 IIIII! 1 I] PUMP WITH AUTOMATIC PRIME DEVICE FIELD OF THEINVENTION The invention relates generally to pumps with priming deviceswhich are disengaged or rendered inoperative whenever the pump is primedto thereby reduce the power requirements for driving the pump. Thepriming device includes a clutch actuated by a pressure signal that isself-induced by the pump. The invention will be hereinafter specificallydescribed as embodied in an electric motor-driven self-priminggas-expelling centrifugal pump and motor unit submerged in a tank ofliquid to be pumped such as the fuel tank of an aircraft fuel system fordelivering vapor-free liquid fuel to the engine In such aircraftinstallations the liquid fuel may be temporarily displaced away from thepump inlet as the aircraft is maneuvered, permitting a conventionalbooster pump to become gas bound or to lose its prime. The built-inpriming device of this invention becomes operative under such conditionsto reprime the pump.

DESCRIPTION OF THE PRIOR ART Self-priming gas-expelling booster pumpsare known in the prior art as for example in the Miles Lowell EdwardsU.S. Pat. No. 3,021,974 dated May 1, I962. In this prior art, however,the priming device for the pump is in operation whenever the pump isdriven thereby wasting power during the major portion of pump operationwhen the pump does not need to be primed. The constantly driven primingdevice has forced the use of larger and heavier pump motors which areespecially undesirable in aircraft installations and which drainadditional power from the electrical system of the aircraft SUMMARY OFTHE INVENTION According to this invention there is now providedmechanism for automatically disengaging the priming device of aself-priming gas-expelling pump whenever the pump is primed. The primingdevice has a clutch drive from the pump motor which is actuated by apressure signal that is self-induced by the pump to overcome a springbias on the clutch. In the priming phrase of operation a spring actsagainst a piston which locks clutching discs to the rotor of the primingdevice so that when the pump motor is energized it will drive both thepriming device and the pump. When the pump is primed and developingliquid pressure in its pumping chamber the piston is actuated under theinfluence of this pressure to overcome the spring bias and unlock theclutching discs so that the priming pump will be disengaged. Thencontinued operation of the motor will be free of the load of the primingdevice. As long the predetermined high pressure remains in the pumpingchamber, the pump will maintain the priming device in disconnectedinoperative condition, but as soon as the pump loses its prime causingthe pressure in the pumping chamber to drop, the spring bias will againact against the piston engaging the clutch and starting the primingdevice into operation for again priming the pump. The cycles will repeateach time the pump loses its prime.

A feature of the invention resides in the provision of a stack ofdiscsto form the clutch permitting the clutch capacity to be varied bychanging the number of discs without materially altering the size of theother parts and thereby making possible the use of the same parts fordifferent prime pump requirements.

Another feature of the invention is the use of high pressure existing inthe volute or pumping chamber of a centrifugal pump for controllingoperation of a priming device for the pump.

It is then an object of this invention to provide a pump equipped with apriming device which is respectively placed into and out of operationwhenever the pump loses and gains its prime.

A further object of this invention is to provide an electricmotor-driven gas-expelling submerged booster pump unit with a built-inpriming device preventing the pump from becoming gas bound and onlyplaced in operation when the pump loses its prime.

Another object of this invention is to provide a submerged centrifugalbooster pump and motor unit with a built-in hydroturbine priming pumpwhich is disengaged from the motor as long as the centrifugal pumpmaintains a desired pressure in its pumping chamber.

Another specific object of this invention is to provide a selfprimingcentrifugal booster pump and electric motor unit with a pump pressureactuated clutch responsive to pump pressure for driving the priming pumponly when the centrifugal pump loses its prime.

Another specific object of this invention is to provide a simplifledclutch composed of a stack of alternate driving and driven clutch discsspring biased into driving position and unloaded by pump pressure todisconnect the rotor of a primimg pump whenever a centrifugal pump isdeveloping a predetermined liquid pressure.

Other and further objects of this invention will become apparent tothose skilled in this art from the following detailed description of theannexed sheets of drawings which, by way of a preferred exampleillustrate one embodiment of the invention.

BRIEF DESCRIPTIOn OF THEDRAWINGS FIG. I is a fragmentary, broken,sectional view of an aircraft fuel tank with the pump and motor unit ofthis invention mounted on a sidewall thereof and shown in elevation;

FIG. 2 is a front end elevational view ofthe pump and motor unit of FIG.I taken along the line II-II of FIG. 1;

FIG. 3 is a longitudinal cross-sectional view ofthe pump and motor unittaken along the line III-III of FIG. 2;

FIG. 4 is a cross-sectional view of the hydroturbine priming pump builtinto the pump and motor unit taken along the line IV-IV of FIG. 3;

FIG. 5 is a plan view of a driving clutch disc for the hydroturbinepump; and

FIG. 6 is a plan view ofa driven clutch disc for the hydroturbine pump.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT The reference numeral 10designates generally a self-priming gas-expelling submerged pump andmotor unit of this invention mounted horizontally in a fuel tank 11 on atank sidewall 12 above the tank bottom wall 13.

The unit 10 has a generally cylindrical casing or housing 14 with aradial mounting flange 15 on one end thereof which is bolted to the tanksidewall 12 by means of capscrews 16 extending through the tank wall andthreaded into bosses 17 around the periphery ofthe flange 15.

The housing I4 has a volute chamber defining portion 18 adjacent the endremote from the flange I5 and surrounding the casing. A reduced diameterthroat portion 19 projects forwardly from the volute portion 18 andterminates in a radial end flange 20 of smaller diameter than the flange15. This flange 20 mounts on elbow pipe 21 having a mounting flange 22on the end thereof secured to the flange 20 as by capscrews 23. The pipe21 extends downwardly in the tank 11 to terminate in an open mouth 24close to the bottom wall of the tank.

The volute chamber defining portion 18 terminates in a flange 25 onwhich is bolted a second elbow pipe 26 through an end flange 27 thereonby means of capscrews 28. The pipe 26 extends through the sidewall 12 ofthe tank and may be sealingly coupled with this sidewall by means notshown.

Fuel from the bottom portion of the tank 13 enters the mouth 24 of thepipe 21 and is pumped by a centrifugal pumping means in the unit I0through the chamber of the volute section 18 and then out of the tankthrough the pipe 26.

As shown in FIG. 3 the cylindrical housing 14 receives a motor and primepump casing 30 through the flange end thereof and this insert casing inturn has a sleeve 31 secured to the leading end thereof by means ofcapscrews 32. The main casing houses an electric motor 33 and is closedby an end cap 34 secured to the flange 15 by means of screws 35. The endcap 34, as shown in FIG. 1, is on the outside of the tank sidewall 12and has a socket 36 with electrical connections for energizing the motor34. The cap 34 has a boss 37 projecting into the motor casing andproviding a support for a motor shaft bearing 38. The motor shaft 39supported at one end by the bearing 38 has a reduced diameter portion 40with flat side faces 41 extending almost to the sleeve 31 and then isfurther reduced in diameter at 42 to terminate in a threaded end 43projecting into the chamber defined by the volute portion 18. The shaftportion 41 is rotatably journaled in a sleeve 44 with a piston head 45fitted in a cylinder 46 defined by the casing portion 30. A spring 47bottomed on the shoulder between the shaft portions 39 and 40 acts onthe piston 45 to bias the sleeve against a stack 48 of clutch elementscomposed of alternate clutch discs or washers 49 having apertures 50therethrough with flat sides 51 riding on the flats 41 of the shaft andclutch discs or washers 52 with cylindrical apertures 53 freelyembracing the shaft portion 40 and having radially projecting lugs 54keyed to the surrounding hub 55 of a hydroturbine pump rotor 56 housedin a double lobe bore 57 provided by the leading end of the casingmember 30. The rotor 56 has radial vanes 58, best shown in FIG 4,coacting with the bore 57 to provide a liquid ring or hydroturbinepriming pump 59.

A plate 60 is clamped between the opposed flanges of the casing sections30 and 31 by the capscrews 32. A sleeve 61 surrounding the shaft portion40 is bottomed at one end on the stack of clutch discs 48 and on theother end by a journal thrust bearing 62 keyed to the shaft portion 42to rotate therewith. The journal thrust bearing 62 is surrounded by thecasing portion 31 is spaced relation and this portion carries carbonthrust rings 63 respectively in face-to-face engagement with a flange62a on one end of the thrust bearing 62 and with a mating thrust ring 64on the shaft portion 42 and clamped between the thrust bearing 62 andthe hub 65 of a centrifugal impeller 66 keyed on the threaded end 43 ofthe shaft.

The impeller 66 has a ring of circumferentially spaced centrifugalimpeller blades 67 depending from a shroud or radial flange 68 on thehub 65. The casing member 31 is contoured to extend through the voluteportion 18 and'fit in the neck 19 where it is sealed to the outerhousing 14 by a seal ring 69. Circumferential slots 70 around theperiphery of the casing 31 communicate the interior of the casing freelywith the volute chamber 71 defined by the housing portion 18 so thatfluid entering the open end ofthe casing 31 is acted on by the impellerblades 67 and centrifugally discharged into the impeller chamber 71 tothe tangential outlet 72 which is connected with the outlet pipe 26described above.

The impeller flange or shroud 68 is ported at 73 close to the hub 65 andthese ports bleed the inlet chamber 74 surrounded by the blades 67 intoa chamber 75 ahead ofthe thrust bearing 64. An annular reservoir 77behind the chamber 75 provided by the casing section 31 around thethrust rings 63 is vented at the top through a passage 76 to a chamberin the housing 14 above the priming pump 59 to receive pumped fluid fromthe priming pump. The chamber 75 is directly ported through a passageway78 with an annular chamber 79 which communicates with an inlet port 80to the hydroturbine pump 59. In addition, the bottom of the reservoir 77is bled into the hydroturbine pump by passages 81 shown in dotted linesin FIG. 3.

Thus, the pumping chamber 74 is vented inwardly of the pumping vanes 67to the hydroturbine priming pump 59. The pump 59 in turn dischargesthrough ports 82 into a chamber surrounding the cylinder 46 which inturn is in open communication with the reservoir 77 and an outletopening 83 in the top of the housing 14. The opening 83 is closed by alight spring-biased flap valve 84 which when opened under the influenceof pressure of fluid from the priming pump will discharge the fluid backto the interior of the tank shown in FIG. 1.

A passageway 85 is provided between the casing 31 and housing 14 fromthe volute chamber 71 to a passageway 86 through the casing 31 whichopens into the cylinder 46 under the piston 45. The piston is thusexposed to the pressure in the volute 71.

It will be understood that the liquid ring or hydroturbine pump 56 iseffective to maintain a ring of liquid in the pump bore 57 under theinfluence of centrifugal force from the hydroturbine vanes 58 and fluidwill be drawn into the pump adjacent the axial central portion of thepump bore through the inlet port 80 to be discharged out of the otherend of the pump through the ports 82. The rotating liquid ring creates asuction chamber in the axial central portion of the pump and thissuction is developed by pulsing the liquid ring radially as it revolvesthrough the lobes of the pump bore. The hydroturbine pump thus acts as avacuum pump effective to draw gases and vapors from the central portionof the pumping chamber ofthe centrifugal pump.

OPERATION When the centrifugal pump is primed and operating, liquid fuelwill be drawn from the bottom portion of the tank 13 through the inletpipe 21 and then discharged through the outlet pipe 26. The liquid inthe volute chamber 71 will be under pressure and this pressure is ventedto the piston 45 for overcoming the spring bias 47 to slide the sleeve44 to the right when viewed in FIG. 3 thereby taking the load offofthestack 48. The driven discs 49 on the shaft portion 40 are thus free toslip relative to the driving discs 52 and the rotor 56 of thehydroturbine pump will be idle.

Whenever the centrifugal pump loses its prime or becomes gas-bound,pressure in the volute chamber 71 will drop whereupon the spring 47 willload the stack ofdiscs 48 causing the rotor 56 to be driven by theelectric motor 33 with the impeller 66 of the centrifugal pump. Drivingof the hydroturbine impeller 56 draws gas or vapor from the pumpingchamber 74 of the centrifugal pump through the ports 73, chamber 75,passageway 78, and chamber 79 into the inlet port 80 of the hydroturbinepump 59. The gases and vapors will be discharged from the pump throughthe outlet 83.

In order to maintain the hydroturbine pump under prime, liquid from thepumping chamber is collected in the reservoir 77 and bleeds to thehydroturbine pump through the passageways 81 if needed. The expelling ofgases and vapors from the pumping chamber 74 by the hydroturbine pumpwill draw liquid into the pumping chamber 74 permitting the centrifugalimpeller blades 67 to act on the liquid and again build up a pressure inthe volute chamber 71 sufficient to overcome the spring bias 47 anddeclutch the rotor 56.

As shown in FIG. 6, the driven clutch members 52 are keyed at 54 intothe hub of the hydroturbine pump rotor 56 while the flats 41 on theshaft portion 40 drives the driving clutch members 49. The stack 48 ofthese alternating driving elements 49 and driven elements 52 can be anydesired height to give the driving couple that is desired for thehydroturbine rotor. Likewise, the spring 47 can be varied to provide thedesired spring bias on this stack 48.

From the above descriptions it will be therefore be understood that thisinvention provides a self-priming gas-expelling centrifugal pump andelectric motor unit with a built-in hydroturbine priming pump which isautomatically rendered inoperative whenever the centrifugal pump isprimed and which is automatically placed into operation whenever thiscentrifugal pump loses its prime.

Any buildup of gases or vapors in the centrifugal pump sufficient tocause the pump to lose its prime will be drawn out of the centrifugalpump under the suction effect of the hydroturbine priming pump.

Although minor modifications might be suggested by those versed in theart, it should be understood that I wish to embody within the scope ofthe patent warranted hereon all such modifications as reasonably andproperly come within the scope of my contribution to the art.

lclaim:

A submerged pump and electric motor unit especially adapted for aircraftfuel tanks which comprises a housing having a mounting flange at one endthereof for attachment to the sidewall of a fuel tank and having aninlet flange on the other end thereof for attachment to an inlet pipe inthe fuel tank, said housing defining a centrifugal pump volute chamberadjacent the inlet pipe mounting flange, a casing in said housing havingan open end' adjacent the mounting flange, and end cap attached to thehousing closing the open end of the casing and providing a shaft bearingsupport, a motor in the casing adjacent said cap, a motor shaft havingone end supported from said cap, said casing providing a piston chamber,a hydroturbine pumpchamber, a bearing chamber, and a pump inlet chambercommunicating with the volute chamber of the housing, said motor shaftextending through said chambers, a centrifugal impeller in said pumpinlet chamber mounted on said motor shaft, thrust hearings in saidbearing chamber surrounding said shaft, a hydroturbine pump rotor insaid hydroturbine pump chamber, a clutch connecting said rotor with saidmotor shaft, a piston in said piston chamber, a spring biasing saidpiston for loading said clutch to couple the motor shaft withthehydroturbine rotor, passageways connecting the volute chamber with thecylinder chamber to act on said piston against the bias of said springfor declutching the hydroturbine rotor from the motor shaft, passagewaysconnecting the pump inlet chamber with the hydroturbine pump chamber,and an outlet in said housing vented to said hydroturbine pump chamberfor releasing gases and vapors from the hydroturbine pump back to thefuel tank whereby said hydroturbine pump will draw gases and vapors fromthe pump inlet chamber and will remain operative only when pressure inthe volute chamber is insufficient to overcome the spring bias on theclutch. 1

2. A submerged gas-expelling centrifugal pump and motor unit adapted foraircraft fuel tank installation which comprises a housing having a firstend flange for mounting the unit in a tank, a second opposite end flangefor mounting an inlet pipe in the tank and a volute portion with atangential outlet terminating in a side flange between said first andsecond flanges for mounting an outlet pipe, an electric motor in saidhousing adjacent said first end flange, a centrifugal pump impeller insaid housing adjacent said second end flange and discharging into saidvolute portion, a liquid ring priming pump in said housing between saidelectric motor and said centrifugal pump impeller, a clutch selectivelycoupling the liquid ring pump with said motor, means venting the,vcentral portion of the area of the housing containing the centrifugalpump impeller with said liquid ring pump to draw gases and vapor fromsaid portion into the liquid ring pump, said housing having a dischargepassage from the liquid ring pump to an outlet for feeding the gases andvapors back to the tank in which the housing is mounted, and meansactuated by pressure in the volute portion for releasing said clutchwhenever liquid is being pumped by said impeller through the voluteportion whereby said liquid ring pump will only be driven by said motorwhen the centrifugal pump loses it prime.

3. The pump of claim 2 including a stack of clutch places coupling theliquid ring pump to the motor and a spring-biased piston loading saidstack and actuated by centrifugal pump pressure against the spring biasfor unloading said stack.

4. An electric-motor-driven centrifugal pump unit which comprises acasing housing an electric motor, a centrifugal pump, and a liquid ringpriming pump having an inlet and an outlet in the casing, means in thecasing venting the centrifugal pump to the priming pump inlet fordrawing gases and vapors from the centrifugal pump, means in the casingventing li uid from the priming pump outlet back to the priming pump inat to prime t e pnmmg pump, an outlet in said casing for gases andvapors expelled from said priming pump, and clutch actuated bypressurefrom the centrifugal pump for disconnecting the priming pumpfrom the electric motor upon development of a predetermined pressure bysaid centrifugal pump whereby the priming pump will be actuated onlywhen the centrifugal pump loses its prime.

5. The pump of claim 4 including a clutch composed of a stack ofalternating driving and driven discs.

6. The combination of claim 4 wherein a liquid-collecting reservoir isprovided between the centrifugal pump and the priming pump formaintaining liquid in the priming pump.

7. In combination a centrifugal pump having a pumping chamberdischarging into a surrounding volute, a liquid ring priming pump havingan inlet vented to said pumping chamber and effective to draw gases andvapors therefrom and an outlet communicating with said inlet, a vent forreleasing gases and vapors from said outlet, a drive shaft, acentrifugal impeller in said pumping chamber driven by said shaft, apriming pump rotor receiving said shaft therethrough, a stack ofalternating discs coupled to said shaft and to said priming pump rotor,means loading said stack to cause the discs coupled to the shaft todrive the discs coupled to the rotor, and means sensitive to pressure inthe volute chamber for unloading said stack to declutch the priming pumprotor whenever the centrifugal pump is operating under primedconditions.

8. The combination of claim 7 wherein the driving discs of the clutchare internally keyed to the shaft and wherein the driven discs of theclutch are externally keyed to the priming pump rotor.

9. The combination of claim 7 including a liquid reservoir between thecentrifugal pump chamber and the priming pump for maintaining liquid inthe priming pump.

10. The combination of claim 7 wherein the priming pump is ahydroturbine pump and a reservoir is provided for supplying liquid tothe hydroturbine pump to maintain a liquid ring therein.

11. A gas-expelling centrifugal pump and liquid ring priming pump saidpump inlet connecting for aircraft fuel tank installation whichcomprises a housing providing a central inlet, a surrounding volutechamber, a liquid ring pump chamber behind the inlet chamber, a clutchchamber behind the liquid ring pump chamber, and an electric motorchamber behind the clutch chamber, a centrifugal pump impeller in saidpump inlet having a ring of vanes discharging centrifugally into thevolute chamber, an electric motor in said motor chamber having a shaftextending through the clutch and liquid ring pump chambers into the pumpinlet, means mounting the centrifugal pump impeller on said shaft insaid pump inlet, ports in the central portion of said pump inletconnecting the central portion of the pump impeller inwardly from thepumping vanes thereon with the liquid ring pump chamber, a liquid ringpump impeller in said liquid ring pump chamber receiving gases andvapors from said ports, a clutch in said clutch chamber connecting saidliquid ring pump impeller with said shaft, means actuated by pressure insaid volute chamber declutching said liquid ring pump impeller from saidshaft, said housing having a chamber surrounding said liquid ring pumpcommunicating with the inlet and outlet of said liquid ring pump wherebyliquid discharged from the ring pump may'flow to the inlet of the liquidring pump for priming the pump, and said housing having a gas and vaporoutlet communicating with said chamber surrounding the liquid ring pumpchamber for releasing gases and vapors from liquid discharged by theliquid ring pump whereby gases and vapors will be drawn from thecentrifugal pump inlet by said liquid ring pump to prevent thecentrifugal pump impeller from becoming gas bound, the liquid ring pumpwill be primed liquid as needed from liquid discharged therefrom and theclutch will couple the liquid ring pump impeller whenever thecentrifugal pump pressure is established.

1. A submerged pump and electric motor unit especially adapted foraircraft fuel tanks which comprises a housing having a mounting flangeat one end thereof for attachment to the sidewall of a fuel tank andhaving an inlet flange on the other end thereof for attachment to aninlet pipe in the fuel tank, said housing defining a centrifugal pumpvolute chamber adjacent the inlet pipe mounting flange, a casing in saidhousing having an open end adjacent the mounting flange, and end capattached to the housing closing the open end of the casing and providinga shaft bearing support, a motor in the casing adjacent said cap, amotor shaft having one end supported from said cap, said casingproviding a piston chamber, a hydroturbine pump chamber, a bearingchamber, and a pump inlet chamber communicating with the volute chamberof the housing, said motor shaft extending through said chambers, acentrifugal impeller in said pump inlet chamber mounted on said motorshaft, thrust bearings in said bearing chamber surrounding said shaft, ahydroturbine pump rotor in said hydroturbine pump chamber, a clutchconnecting said rotor with said motor shaft, a piston in said pistonchamber, a spring biasing said piston for loading said clutch to couplethe motor shaft with the hydroturbine rotor, passageways connecting thevolute chamber with the cylinder chamber to act on said piston againstthe bias of said spring for declutching the hydroturbine rotor from themotor shaft, passageways connecting the pump inlet chamber with thehydroturbine pump chamber, and an outlet in said housing vented to saidhydroturbine pump chamber for releasing gases and vapors from thehydroturbine pump back to the fuel tank whereby said hydroturbine pumpwill draw gases and vapors from the pump inlet chamber and will remainoperative only when pressure in the volute chamber is insufficient toovercome the spring bias on the clutch.
 2. A submerged gas-expellingcentrifugal pump and motor unit adapted for aircraft fuel tankinstallation which comprises a housing having a first end flange formounting the unit in a tank, a second opposite end flange for mountingan inlet pipe in the tank and a volute portion with a tangential outletterminating in a side flange between said first and second flanges formounting an outlet pipe, an electric motor in said housing adjacent saidfirst end flange, a centrifugal pump impeller in said housing adjacentsaid second end flange and discharging into said volute portion, aliquid ring priming pump in said housing between said electric motor andsaid centrifugal pump impeller, a clutch selectively coupling the liquidring pump with said motor, means venting the central portion of the areaof the housing containing the centrifugal pump impeller with said liquidring pump to draw gases and vapor from said portion into the liquid ringpump, said housing having a dischaRge passage from the liquid ring pumpto an outlet for feeding the gases and vapors back to the tank in whichthe housing is mounted, and means actuated by pressure in the voluteportion for releasing said clutch whenever liquid is being pumped bysaid impeller through the volute portion whereby said liquid ring pumpwill only be driven by said motor when the centrifugal pump loses itprime.
 3. The pump of claim 2 including a stack of clutch placescoupling the liquid ring pump to the motor and a spring-biased pistonloading said stack and actuated by centrifugal pump pressure against thespring bias for unloading said stack.
 4. An electric motor-drivencentrifugal pump unit which comprises a casing housing an electricmotor, a centrifugal pump, and a liquid ring priming pump having aninlet and an outlet in the casing, means in the casing venting thecentrifugal pump to the priming pump inlet for drawing gases and vaporsfrom the centrifugal pump, means in the casing venting liquid from thepriming pump outlet back to the priming pump inlet to prime the primingpump, an outlet in said casing for gases and vapors expelled from saidpriming pump, and clutch actuated by pressure from the centrifugal pumpfor disconnecting the priming pump from the electric motor upondevelopment of a predetermined pressure by said centrifugal pump wherebythe priming pump will be actuated only when the centrifugal pump losesits prime.
 5. The pump of claim 4 including a clutch composed of a stackof alternating driving and driven discs.
 6. The combination of claim 4wherein a liquid-collecting reservoir is provided between thecentrifugal pump and the priming pump for maintaining liquid in thepriming pump.
 7. In combination a centrifugal pump having a pumpingchamber discharging into a surrounding volute, a liquid ring primingpump having an inlet vented to said pumping chamber and effective todraw gases and vapors therefrom and an outlet communicating with saidinlet, a vent for releasing gases and vapors from said outlet, a driveshaft, a centrifugal impeller in said pumping chamber driven by saidshaft, a priming pump rotor receiving said shaft therethrough, a stackof alternating discs coupled to said shaft and to said priming pumprotor, means loading said stack to cause the discs coupled to the shaftto drive the discs coupled to the rotor, and means sensitive to pressurein the volute chamber for unloading said stack to declutch the primingpump rotor whenever the centrifugal pump is operating under primedconditions.
 8. The combination of claim 7 wherein the driving discs ofthe clutch are internally keyed to the shaft and wherein the drivendiscs of the clutch are externally keyed to the priming pump rotor. 9.The combination of claim 7 including a liquid reservoir between thecentrifugal pump chamber and the priming pump for maintaining liquid inthe priming pump.
 10. The combination of claim 7 wherein the primingpump is a hydroturbine pump and a reservoir is provided for supplyingliquid to the hydroturbine pump to maintain a liquid ring therein.
 11. Agas-expelling centrifugal pump and liquid ring priming pump said pumpinlet connecting for aircraft fuel tank installation which comprises ahousing providing a central inlet, a surrounding volute chamber, aliquid ring pump chamber behind the inlet chamber, a clutch chamberbehind the liquid ring pump chamber, and an electric motor chamberbehind the clutch chamber, a centrifugal pump impeller in said pumpinlet having a ring of vanes discharging centrifugally into the volutechamber, an electric motor in said motor chamber having a shaftextending through the clutch and liquid ring pump chambers into the pumpinlet, means mounting the centrifugal pump impeller on said shaft insaid pump inlet, ports in the central portion of said pump inletconnecting the central portion of the pump impeller inwardly from thepumping vanes thereon with the liquid ring pump chamber, a liquid ringpump impeller in said liquid riNg pump chamber receiving gases andvapors from said ports, a clutch in said clutch chamber connecting saidliquid ring pump impeller with said shaft, means actuated by pressure insaid volute chamber declutching said liquid ring pump impeller from saidshaft, said housing having a chamber surrounding said liquid ring pumpcommunicating with the inlet and outlet of said liquid ring pump wherebyliquid discharged from the ring pump may flow to the inlet of the liquidring pump for priming the pump, and said housing having a gas and vaporoutlet communicating with said chamber surrounding the liquid ring pumpchamber for releasing gases and vapors from liquid discharged by theliquid ring pump whereby gases and vapors will be drawn from thecentrifugal pump inlet by said liquid ring pump to prevent thecentrifugal pump impeller from becoming gas bound, the liquid ring pumpwill be primed liquid as needed from liquid discharged therefrom and theclutch will couple the liquid ring pump impeller whenever thecentrifugal pump pressure is established.